1. Field of the Invention
The present invention relates to a curvature limiter device forming a tubular member suitable for adopting a rectilinear position or a position of variable curvature while preventing a radius of curvature that is less than a given minimum radius of curvature R0, and thus allowing controlled variable curvature or no curvature in a flexible line that is threaded inside said tubular member.
The present invention also relates to a bottom-to-surface connection installation between a) undersea equipment, such as an undersea well head or the end of an undersea pipe resting on the sea bottom, and b) a floating support on the surface, the installation including an undersea flexible line of curvature that is locally controlled by a curvature limiter.
More particularly, the invention relates to an installation comprising a hybrid tower made up of a flexible pipe connected to a rigid riser column or vertical riser, having its bottom end connected to the top end of a said piece of equipment or to a said end of an undersea pipe resting on the sea bottom, said tower being coupled to at least one said flexible line passing through a curvature limiter attached to said riser column and/or in suspension attached to a subsurface float.
2. Detail Description of the Prior Art
The term undersea flexible line is used herein to mean a line that is easy to curve in order to obtain a minimum radius of curvature of a few meters, and in particular in the range 0.5 meters (m) to 5 m while remaining in the elastic range, as contrasted to a rigid steel pipe having a solid wall and for which the radius of curvature in the elastic domain is several tens or even hundreds of meters. By way of example, such flexible lines may be:
a) a flexible pipe for transferring a petroleum fluid, such as the hoses fabricated by Coflexip (France) and used in particular in the above-mentioned hybrid towers, conventionally being constituted by an internal tube of flexible polymer material reinforced by braided composite or metal wire reinforcement forming spiral-wound sheaths, and where appropriate a plurality of intermediate tubes or outer tubes between layers of said reinforcement;
b) pipes or cables for transferring energy or information such as electric cables, control cables, hydraulic fluid transfer pipes feeding hydraulic equipment such as actuators, or pipes containing optical fibers; or indeed
c) umbilicals, i.e. pipes made up of a plurality of such electric cables, hydraulic pipes, control cables, and/or optical fibers.
The technical field of the invention is more particularly the field of fabricating and installing production riser columns (or “risers”) for conveying oil, or gas, or any other soluble or meltable material, or a suspension of mineral materials, from an underwater well head to a floating support in order to develop production fields installed off-shore, out at sea. The main and immediate application of the invention lies in the field of oil production.
In general, a floating support has anchor means to keep it in position in spite of the effects of currents, winds, and swell. It also generally includes means for storing and processing oil together with off-loading means for off-loading oil to tankers that call at regular intervals in order to remove the production. The common term for such supports is floating production storage and off-loading supports, and they are referred to throughout the description below by the initials FPSO.
Hybrid tower type bottom-to-surface connections for an undersea pipe resting on the sea bed are known and they comprise:                a substantially vertical steel riser having its bottom end anchored to the sea bottom and connected to a said pipe resting on the sea bottom; and        a connection pipe, generally a flexible connection pipe in continuity of curvature as a dipping catenary between the top end of said riser and a floating support on the surface, said flexible connection pipe possibly including buoyancy elements along a fraction of its length. The term “dipping catenary” is used herein to mean a curve having the geometrical shape known as a “catenary” with a bottom point of inflection.        
Connections of this type are described more particularly in patent WO 2009/138609. Patent WO 00/49267 in the name of the Applicant describes a variant hybrid tower having a plurality of vertical rigid pipes that are secured to one another at various depths.
Additional bottom-to-surface flexible connections of the electric cable or electrohydraulic umbilical type are added after the hybrid tower has been installed, said additional bottom-to-surface connections generally extending continuously from the FPSO all the way to their destination, i.e. for an undersea well head situated several kilometers away from said FPSO.
In this context, it is recalled that the essential function of these dipping catenary portions of flexible lines is to absorb, at least in part, the movements of the rigid pipes to which such a line is fastened or attached, and/or the movements of the floating supports to which said flexible line is connected, by mechanically decoupling the movements respectively of said rigid pipes and of said floating supports. However another function is also to reduce the traction forces exerted by the flexible line on the undersea equipment on the sea bottom to which it is connected, where applicable.
Document FR 09/58096 in the name of the Applicant, filed on Nov. 17, 2009, discloses bottom-to-surface installations in which a plurality of flexible lines comprise line portions in the form of dipping catenary curves that are obtained by suspending a portion of said flexible line from a buoy at an intermediate depth between the bottom and the surface. For that purpose, said intermediate portion of flexible line or pipe is caused to pass via a trough defining a rigid bearing surface of convex curved shape.
Such a trough serves to give the portion of flexible pipe that it supports curvature that is controlled so as to avoid excessive curvature that would damage the hose irremediably. However installing such troughs and also flexible pipes over such troughs requires maneuvers that are complicated and lengthy and therefore expensive.
In FR 2 889 557, proposals are made to use a curvature limiter device that is constituted by a tubular member through which the pipe is threaded, which member is suitable for being pre-installed around the pipe either in a factory, or else on board a laying ship. The curvature limiter device is laid together with the pipe threaded through said curvature limiter device, the device presenting a rectilinear shape and outside diameter dimensions that are compatible with the dimensions of hatches for passing the pipe on board the laying ship, in particular at the bases of J-lay towers, that are used for unwinding and laying pipes at sea from a laying ship. It then suffices to suspend or attach the curvature limiter at an intermediate depth between the bottom and the surface, and then give it a curved shape with a convex side facing upwards so as to create on at least one of its sides a portion of flexible line or pipe that has the shape of a dipping catenary. That flexible tubular member constituting said curvature limiter as described in FR 2 889 557 comprises a plurality of tubular sleeves arranged end-to-end in an axial longitudinal direction and connected together loosely by annular members. Said sleeves and said annular members present non-cylindrical longitudinal ends that engage mutually in one another without being fastened to one another. More precisely, the sleeves present collars at each end that are curved radially outwards to form shoulders while the annular members present at each of their ends inward radial curvature forming shoulders in the opposite direction and suitable for co-operating by mutual engagement with the shoulders at the ends of said sleeves.
The annular sleeves and members as mutually engaged in this way in succession constitute a hinged assembly allowing movement between its various elements, both in the axial longitudinal directions of the sleeves and in lateral directions that are perpendicular to said axial longitudinal directions. Such movement enables the tubular member to be curved, with said curvature being limited by the maximum amount of movement allowed on upper faces of said annular sleeves and members when their bottom faces are at minimum movement.
That type of curvature limiter thus allows the flexible line or pipe to take up varying curvature, including a rectilinear position, while preventing the curvature of said flexible line dropping below a determined minimum radius of curvature.
Such curvature devices of the mutually engaged vertebrae type as described in patents FR 2 889 557 and U.S. 2010/0228295 have been known for a long time and were developed and used initially on the Chevron-Ninian site in the North Sea in 1978.
GB 2 334 048 also describes a curvature restrictor for a flexible pipe of the type comprising alternating and mutually engaged half-vertebrae.
That type of curvature limiter presents certain drawbacks. Firstly, they are difficult and expensive to fabricate since they require mutual engagement of parts that have been fabricated on a lathe with relatively high precision, and thus parts that are relatively expensive to fabricate. Similarly, assembling the various annular sleeves and members is relatively difficult and complex to achieve.
Furthermore, that curvature limiter device does not provide curvature that is stable and accurate, since its stability is associated solely with its own weight. Furthermore, that curvature limiter device can be deformed, particularly if it is pulled laterally in a direction perpendicular to its mean plane of curvature because the various mutually engaged unit elements are independent of one another and can be subjected to lateral movements relative to one another, which constitutes a drawback, in particular when it is desired to use a plurality of flexible lines and thus a plurality of curvature limiters that are located close together side-by-side, and in particular that are arranged on two opposite faces of a float or of a rigid column as described in FR 09/58096.
Another drawback of the curvature limiter device described in FR 2 889 557 stems from that fact that it limits the curvature of the flexible line in all directions, which, under certain circumstances, can impede or even prevent the flexible line fitted with said curvature limiter device being wound around a storage spool on board a laying ship.
Another drawback of the curvature limiter device described in FR 2 889 557 is that the pipe is clamped inside the sleeves to keep the curvature limiter in a position that is stationary relative to the pipe, which requires said curvature limiter device to be installed around a unit pipe element only after causing a pipe portion of corresponding length to pass through and be laid as a function of the depth at which said curvature limiter device is to be attached or suspended between the bottom and the surface.